1. Technical Field
The present invention relates to a semiconductor device used in a semiconductor integrated circuit and having a protective resistance element, and a method for manufacturing the same.
2. Related Art
The smaller the size of input-output transistors become because of trends toward microminiaturization of semiconductor integrated circuits, the more easily the breakdown of gate oxide films due to electrostatic discharge (ESD) is caused. One of anti-electrostatic measures is to provide an input-output protective device upstream of input-output transistors. For example, the input-output protective device includes a parasitic bipolar transistor, and dissipates a current by bipolar operation to protect input-output transistors when an excess voltage such as ESD is transmitted from pads (for example, refer to Japanese Unexamined Patent Publication No. 2001-36006 (p. 3-4, FIG. 1)).
In some input-output transistors having a certain size or structure, the provision of an input-output protective device is not required and a resistance element, which is inexpensive, contributes to electrostatic protection. Even in a structure including an input-output protective device, it is important to provide a resistance element between pads and the input-output protective device, or upstream of input-output transistors, for inducing an adequate voltage drop. Typical one of various configurations of these resistance elements is a diffused resistor (well resistor). At present, such a diffused resistor does not have a preferable structure since anti-electrostatic measures are insufficient, and therefore there is a room for improvement.
Resistance elements made up of the above diffused resistor (well resistor) are formed simultaneously with a forming step of other transistor elements or the like. Therefore, the impurity concentration of the resistance elements is not adjusted independently in general. Thus the resistance value is adjusted with the length and width of a diffused region between coupled wires. This sometimes makes it difficult to form a desired resistance element within a limited region. Also, since discharge channel is along a single direction, charges applied excessively because of ESD or the like may damage resistors.
The present invention is made in consideration of the above circumstances, and is intended to provide a semiconductor device having an inexpensive and high performance resistance element that can ensure, within a more reduced area, a large area for discharging excess charges due to ESD or the like. The present invention is also intended to provide a method for manufacturing the semiconductor device.